The present invention generally relates to optoelectronic devices and more specifically relates to a method of manufacturing optoelectronic devices with improved light extraction.
Optoelectronic devices such as light emitting diodes (xe2x80x9cLEDsxe2x80x9d) and laser diodes (xe2x80x9cLDsxe2x80x9d) are typically comprised of semiconductor material including thin epitaxial layers of two opposite conductivity types, referred to as p-type and n-type. The layers are disposed in a stack, one above the other, with one or more layers of n-type material in one part or region of the stack and one or more layers of p-type material in another part or region of the stack opposite from the n-type material. For example, the various layers or epitaxial layers may be deposited or grown in sequence on a substrate to form a wafer. The wafer is then separated or cleaved apart to form individual dies which constitute separate individual optoelectronic devices such as separate LEDs or LDs. The junction between the p-type and n-type material may include directly abutting p-type and n-type layers, or may include one or more intermediate layers which may be of any conductivity type or which may have no distinct conductivity type.
In operation, electric current passing through the diode is carried principally by electrons in the n-type layers and by electron vacancies or xe2x80x9cholesxe2x80x9d in the p-type layers. The electrons and holes move in opposite directions toward the junction and recombine with one another at the junction. Energy released by electron-hole recombination is emitted as light. As used in this disclosure, the term xe2x80x9clightxe2x80x9d includes radiation in the infrared and ultraviolet wavelength ranges, as well as radiation in the visible range. The wavelength of the light depends on factors including the composition of the semiconductor materials and the structure of the junction.
Electrodes are connected to the n-type and p-type layers near the top and bottom of the stack. The materials in the electrodes are selected to provide low-resistance interfaces with the semiconductor materials. The electrodes, in turn, are provided with pads suitable for connection to wires or other conductors which carry current from external sources. The pad associated with each electrode may be a part of the electrode, having the same composition and thickness of the electrode, or may be a distinct structure which differs in thickness, composition, or both from the electrode itself.
Some optoelectronic devices such as LEDs or LDs have electrodes on the bottom surface of the bottom epitaxial layer. For example, the various epitaxial layers may be deposited or grown in sequence on an electrically conductive substrate, and the substrate may be left in place on the bottom surface to act as a bottom electrode after individual dies are separated from the wafer. Typical LEDs or LDs formed from certain semiconductor materials, however, normally are grown on nonconductive substrates to promote proper formation of the semiconductor layers. Thus, if a nonconductive substrate is left in place, an electrode cannot be provided on the bottom surface of the bottom layer. For example, gallium nitride-based materials such as GaN, AlGaN, InGaN and AlInGaN are used to form LEDs or LDs emitting light in various wavelength ranges including blue and ultraviolet. These materials typically are grown on insulating substrates such as sapphire.
LEDs or LDs incorporating an insulating substrate must include a bottom electrode at a location on the stack above the substrate but below the junction. Typically, the upper layer or layers of the stack are removed after formation of the stack in a region covering part of the area of each die, so as to provide an upwardly-facing lower electrode surface on a layer at or near the bottom of the stack in each die. This leaves a region referred to as a xe2x80x9cmesaxe2x80x9d projecting upwardly from the lower electrode surface and covering the remaining area of the die. The area of the die occupied by the lower electrode surface does not emit light. Thus, it is desirable to keep the horizontal extent of this inactive area as small as possible.
Additionally, LEDs or LDs may be flip chip packaged, leaving the substrate material on the LED or LD die intact. Some optoelectronic devices, such as gallium-nitride based LEDs or LDs, are deposited or grown on sapphire substrates. Sapphire and other transparent substrate materials cause internal reflections between the substrate and the gallium-nitride based LED or LD, limiting light extraction from the device. Therefore, it is desirable to remove the substrate to improve light extraction from the device. Known methods of substrate removal include using a laser to delaminate the substrate from the semiconductor material of the wafer. Delamination of the substrate from the semiconductor material, however, causes problems such as cracking of the epitaxial layers due to thermal and mechanical stress during exposure to the laser. The epitaxial layers, therefore, are weakened and more prone to further chipping and damage upon separation into individual dies.
Lapping and polishing may also be utilized to remove the substrate from semiconductor material of the wafer. Lapping and polishing is disadvantageous because it is a mechanical process which exerts stress on the wafer and the active layers of the device. This stress can lead to immediate failure during the lapping as well as long term reliability issues. Moreover, lapping and polishing is extremely difficult to perform on a free, or insufficiently supported structure, resulting in low yields. Additionally, lapping and polishing are difficult to control. It is difficult to remove the substrate via lapping and polishing at the interface between the substrate and the epitaxial layers. Therefore, the last 10-20 microns of substrate thickness have to be removed by selective chemical etching which is not available for sapphire.
An improved method of manufacturing an optoelectronic device having improved light extraction is needed that includes removing the substrate from the epitaxial layers.
The present invention provides an improved method of manufacturing optoelectronic devices such as LEDs and LDs having improved light extraction. A method according to one aspect of the invention includes providing a substrate having first and second major surfaces and growing epitaxial layers on the first major surface of the substrate. The epitaxial layers desirably include a first region of a first conductivity type and a second region of a second conductivity type and a light-emitting p-n junction between the regions. The region of first conductivity type may be p-type, and the region of second conductivity type may be n-type.
Separations are formed through the epitaxial layers to the first major surface of the substrate to provide a structure including a plurality of individual dies or individual LED or LD devices on the first major surface of the substrate. The structure including the plurality of individual dies is preferably mounted to a submount to expose the second major surface of the substrate. The substrate is then removed from the structure.
For example, using a laser to delaminate the substrate from the structure. The laser used in the removal step desirably operates at a wavelength that is absorbed by the epitaxial layers and is not absorbed by the substrate.
Additionally, the removal step may include mechanically abrading the substrate. Such mechanical abrasion may involve lapping and polishing the substrate to expose epitaxial layers.
In another aspect of the invention, after the substrate material is removed, an index matching material having a refractive index greater than or, most preferably, substantially equal to the refractive index of the second region of the epitaxial layers is attached in place of the substrate. According to one embodiment of the invention, the index matching material is lens-shaped to improve light extraction from the device. The index matching material may be attached to the second region via dispensing the index matching material as a liquid, gluing or using epoxy to attach the index matching material.
The index matching material has a refractive index substantially equal to the geometric mean of the refractive index of the second region and the refractive index of the surrounding light guiding medium.
The index matching material is attached to the device via sputtering, spinning from a liquid, evaporation deposition, or through plasma enhanced chemical vapor deposition. The index matching material may be, for example, a polymer such as epoxy or silicone. In addition the index matching material may be glass, and may contain phosphor.
The separations desirably are formed using a laser, a saw blade, or dry or wet etching. The separations may be configured in various ways, according to the desired shape of the final optoelectronic device. Thus, the individual dies may be, for example, square-shaped or rectangular-shaped.
In one embodiment of the invention, the submount is comprised of, for example, sapphire, silicon, silicon carbide, ceramics and polyimide. The structure may be mounted to the submount using an adhesive. In another embodiment of the invention, the submount may be a permanent mount such as a circuit board, driver or an electronic circuit.
According to yet another aspect of the invention, the individual dies may be detached from the submount after the substrate has been removed. Thus, further processing may be done to the individual dies. The detaching may occur via thermal heating of the adhesive or using a solvent on the adhesive.
In preferred embodiments o the present invention removal of the substrate reduces internal reflections in the optoelectronic device and improves the light extraction. Substrate removal is facilitated by cutting separations through the LED or LD epitaxial layers to the surface of the substrate prior to removal of the substrate. Moreover, the addition of the index matching material further enhances light extraction of the device.